Decanter centrifuge with a screw conveyor having a varying pitch

ABSTRACT

The decanter centrifuge comprises a screw conveyor having a body ( 4 ), which carries a screw comprising one or more flights ( 7, 7 ′) and having a nominal transport speed varying along the longitudinal axis. An inlet ( 6 ) is provided in the screw conveyor for the material to be separated. The screw conveyor is provided with a baffle ( 8, 8 ′) dividing the separation chamber in a substantially cylindrical separation part ( 17 ) and an at least partially conical discharge part ( 18 ). Immediately upstream of the baffle ( 8, 8 ′) a transition part ( 19 ) is provided between the separation part ( 17 ) and the discharge part ( 18 ), the screw conveyor ( 3 ) having a bigger nominal transport speed in the transition part ( 19 ) than in the separation part ( 17 ) immediately before the transition part ( 19 ), the change of the nominal transport speed being established by a change ( 21 ) of the screw pitch.

FIELD OF THE INVENTION

The present invention relates to a decanter centrifuge for separation ofa supplied material in a light phase and a heavy phase, comprising anelongate bowl arranged for rotation about its longitudinal axis, saidbowl having a separation chamber, a screw conveyor being provided in theseparation chamber and being coaxial with the bowl, said screw conveyorcomprising a body, which carries a screw comprising one or more flightsand having a nominal transport speed varying along the longitudinalaxis, an inlet with at least one inlet opening in the screw conveyor forsupply of the material to the separated, and at least one dischargeopening for the heavy phase in the bowl at one end of the screwconveyor, in which the screw conveyor is made to rotate relative to thebowl in view of conveying the heavy phase towards the discharge openingsfor the heavy phase, and in which the screw conveyor is provided with abaffle positioned between the inlet openings and the discharge openings,said baffle dividing the separation chamber in a substantiallycylindrical separation part and an at least partially conical dischargepart, the discharge openings for the heavy phase being positioned in thedischarge part, the inlet openings being positioned at the opposite sideof the baffle relative to said discharge openings.

DESCRIPTION OF RELATED ART

A decanter centrifuge of this kind is known from WO-A-97/22411, whichdiscloses a decanter centrifuge having a baffle shaped as a ribextending from the upstream side of a screw turn as a part of a turnhaving a bigger pitch than the screw to the downstream side of a screwturn at an axial distance from its starting point.

U.S. Pat. No. 3,934,792 discloses a decanter centrifuge having a baffleextending axially from the upstream side of the screw turn to thedownstream side of the adjacent screw turn. A similar baffle isdescribed in U.S. Pat. No. 5,653,673.

U.S. Pat. No. 3,885,734, U.S. Pat. No. 4,245,777 and U.S. Pat. No.4,381,849 disclose baffles extending tangentially around the screwconveyor.

The flight or flights of a screw conveyor defines/define a passagewaybetween adjacent turns, through which material flows during the runningof the decanter centrifuge. A baffle is in general a member barring apart of the cross section of the passageway at a distance from theinterior wall of the bowl. If only one flight is provided, it forms asingle passageway winding around the body of the screw conveyor, and thebaffle will comprise a single member. If several flights are provided, asimilar number of passageways will be defined between them, and thebaffle will therefore comprise a member in each passageway.

In a decanter centrifuge a separation of the heavy phase and the lightphase takes place in the separation part, whereby the light phase may bewater and the heavy phase may be sludge to be drained off. The drainedoff sludge is conveyed by the screw through the bowl to the baffle,under the baffle, i.e. between the baffle and the interior wall of thebowl, and to the discharge openings, where the comparatively dry sludgeleaves the centrifuge, the baffle preventing the water or the lightphase from reaching the discharge openings for the heavy phase.

The separation part and the part of the screw present therein aredesigned with a view to obtaining the biggest possible efficiency of thedrainage. However, an accumulation of the heavy phase immediately beforethe baffle may occur, partly on account of the throttling of the flowarea of the heavy phase caused by the baffle, partly on account of thereduced area in the conical discharge part, which acts backwards in sucha manner that the separation process in the separation part does not getthe intended course, which moreover entails a poorer process economy anda poorer drainage.

It is the object of the invention to reduce this problem.

SUMMARY OF THE INVENTION

This object is according to the invention met in that immediatelyupstream of the baffle, seen in relation to the transport direction, atransition part is provided between the separation part and thedischarge part, and that the screw conveyor has a bigger nominaltransport speed in the transition part than in the separation partimmediately before the transition part, the change of the nominaltransport speed of the screw from the nominal transport speed in theseparation part immediately before the transition part to the highernominal transport speed in the transition part being established by achange of the screw pitch.

By nominal transport speed for the screw is to be understood the speed,at which a given part of the screw would convey the heavy phase withoutdisturbance from the surrounding parts of the screw, like for instancedownstream accumulation of heavy phase. The nominal transport speeddepends in a non-linear way on the screw pitch and is highest at a pitchangle of approx. 450° relative to the tangential direction.

By designing the screw in accordance with the invention is attained thataccumulation of the heavy phase in the discharge part will not takeplace to the same degree, as would otherwise be the case. Letting theincrease of the transport speed take place before the baffle minimizesthe risk of pulling to pieces the heavy phase, which at this point hasthe character of a coherent cake, which would entail a risk of the lightphase breaking through to the discharge part, which must be avoided, asit is tantamount to a re-wetting of the heavy phase just drained.

The change of the screw pitch may be abrupt, which may be convenientfrom a constructional point of view, but the change of the screw pitchmay alternatively be gradual.

In a preferred embodiment the pitch angle of the screw in the separationpart is considerably smaller than 45° relative to the tangentialdirection, and the change of the screw pitch from the separation part tothe transition part is an increase. This increase is preferably 40–80%.

In another embodiment the pitch angle of the screw in the separationpart is considerably bigger than 45° relative to the tangentialdirection, and the change of the screw pitch is a decrease from theseparation part to the transition part.

To obtain full effect of the invention the heavy phase, which isconveyed towards the baffle, should be conveyed at the increased speedover the whole peripheral extension of the baffle. Therefore, the screwhas the bigger nominal transport speed over at least ⅓×1/n of a turnbefore the baffle, preferably over approximately ⅔×1/n of a turn, nbeing the number of flights, corresponding to an axial length of ⅓ andpreferably ⅔, respectively, of the pitch in the transition part, ifthere is only one flight, or the axial distance between two adjacentturns, if several flights are present.

In an embodiment, in which the baffle has an axial extension, the borderbetween the discharge part and the transition part is considered to beat the centre point of the axial extension of the baffle.

The inlet is preferably placed upstream of the transition part in theseparation part itself. In this way the risk of turbulence, on accountof the change of speed, disturbing the inlet flow is eliminated.

The screw pitch may be increasing in the separation part in a directionaway from the transition part. In this manner known per se a decreasingconcentration of the heavy phase in a direction away from the inlet andthe discharge part is compensated for.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be explained in detail in the following by meansof some examples of embodiments and with reference to the drawings, inwhich

FIG. 1 in a somewhat schematic form shows a longitudinal section of aknown decanter centrifuge having a bowl with a screw conveyor with anannular baffle disc, and

FIG. 2 a screw conveyor in a first embodiment of the invention,

FIG. 3 a screw conveyor according to a second embodiment, and

FIG. 4 a screw conveyor in a third embodiment according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The decanter centrifuge 1 in FIG. 1 has a hollow bowl 2 with aseparation chamber containing a screw conveyor 3 having a body 4 with ascrew with a flight 7, which is wound in a number of turns. The body 4is substantially cylindrical and has a conical part 5 at one end. In thescrew conveyor 3 inlet openings 6 for the material to be separated areprovided, and in the bowl 2 discharge openings 14 for the separatedheavy phase are provided. As indicated in the figure, the light phase 12will be positioned closest to the body of the screw conveyor 4, whereasthe heavy phase 13 is positioned at the interior side of the bowl 2. Thelight phase is taken away via a discharge edge 10 on the bowl. The heavyphase is conveyed by the screw turn towards the discharge openings 14 inthe bowl at its conical end. The figure shows a baffle 8 comprising anannular disc, which is perpendicular to the longitudinal axis or axialdirection of the screw conveyor.

FIG. 2 shows a screw conveyor 3, which as the screw conveyor in FIG. 1is provided with a baffle 8 in the form of an annular disc and an inletopening 6. FIG. 2 shows by broken lines the enveloping surface for thescrew turns of the flight 7. The enveloping surface comprises acylindrical part 15 and a conical part 16. The enveloping surfacecorresponds with a suitable clear to the shape of the bowl, in which thescrew conveyor is to be mounted.

The baffle 8 is positioned near the transition between the conical part16 and the cylindrical part 15, and it divides substantially thecentrifuge or the separation chamber in a cylindrical separation part 17and a conical discharge part 18. In the embodiment the discharge part 18comprises, however, a small portion of the cylindrical part 15.

The pitch of the screw turns varies along the screw conveyor 3 in itsaxial direction 20. Thus, there is at a point or at an axial position 21an abrupt leap of the pitch of approximately 58%. The position 21 marks,on account of the change-constituted by the leap, a dividing linebetween the separation part 17 and a transition part 19 between theseparation part 17 and the discharge part 18.

The pitch is in the embodiment constant from the position 21 to thedischarge openings for the heavy phase.

The pitch of the screw turns in the separation part 17 is in thisexample decreasing in the axial direction 20 such that the pitch issmallest immediately before the transition part 19. The inlet 6 issituated in the separation part 17 shortly before the transition part19.

FIG. 3 shows another embodiment having a baffle 8 extending axially. Theflight 7 of the screw conveyor 3 has in the position 21 a leap of thepitch, which is consequently bigger in the transition part 19 than inthe separation part 17. In the separation part 17 the pitch is constant.On account of the axial extension of the baffle 8, the dividing linebetween the transition part 19 and the discharge part 18 is consideredto lie at the axial centre point 23 of the baffle. As the position 21 issomewhat downstream of the starting point 24 of the baffle, the position21 will lie slightly more than a half pitch before the centre point 23of the baffle. The pitch of the flight 7 is, in the screw conveyorsdescribed up till now, equal to the axial dimension of the passageway 25formed between the adjacent turns of the flight 7, and the pitch angleof the flight 7 in the separation part 17 is substantially smaller than45° relative to the tangential direction.

FIG. 4 shows an embodiment, in which the screw of the screw conveyor 3has three flights 7′ having a pitch angle substantially bigger than 45°relative to the tangential direction in the separation part 17. At anaxial position 21′ the pitch is changed, the pitch angle being changedin direction of 45°, following which the nominal transport speedincreases.

At the position 21′ a baffle member 8′ extends from each flight 7′, saidbaffle member extending as a part of a turn having a higher pitch thanthe flights 7′ in the transition part 19 and the discharge part 18, butwith the same rotational direction such that the baffle members 8′extend from a downstream side surface 26 of a flight 7′ to an upstreamside surface 27 of an adjacent flight 7′. In the embodiment shown inFIG. 4, the baffle members 8′ have the same pitch as the flights 7′ inthe separation part, but that need not be the case.

The baffle members 8′ extending from the position 21′ and having a pitchsmaller than 90° (axial direction), and the dividing line between thetransition part 19 and the discharge part 18 being counted to lie at theaxial centre point 23 of the baffle members, the leap regarding thenominal transport speed occurs more than ⅙ (½×⅓ (3=the number offlights)) of a flight upstream of the transition part corresponding tomore than half of the axial extension of a passageway 25 between twoadjacent flights 7′ in the transition part.

A centrifuge with a screw conveyor according to the invention works inthe following way.

Material to be separated, for example aqueous sludge, is led into theseparation chamber through the inlet 6. The sludge flows through thepassageway 25 established by the flight 7 of the screw turn, or thepassageways 25 established by the flights 7′, towards the left of thefigures. On its way, the heavy phase sediments, i.e. the sludge, asindicated in FIG. 1.

The screw conveyor 3 pulls on account of its rotation relative to thebowl 2 the sedimented sludge to the right of the figures (downstreamdirection). The sludge is compressed in the separation part 15 up to theaxial position 21. At this point, the sludge forms a coherent,comparatively dry cake.

From the position 21 the sludge is accelerated on account of the changeof the pitch of the flight 7 or the flights 7′. The position 21 is inthe embodiment in FIG. 2 positioned approximately ⅔ turn before theintersection point 21 of the screw turn 7 with the baffle 8corresponding to an axial distance between the position 21 and the point22 of ⅔ of the pitch of the screw turn or the axial dimension of thepassageway at this point. In the embodiments in FIGS. 3 and 4, theposition 21 is positioned a little bit more than ½ time the axialdimension of the passageway 25 upstream of the axial centre point 23 ofthe baffle 8 or the baffle members 8′. In this manner the changing pointof the transport speed is situated sufficiently far from the baffle 8,8′ to convey the sludge along the periphery of the entire baffle at theincreased speed.

The space between the periphery of the baffle 8 and the interior wall ofthe bowl 2 is smaller than the thickness of the sludge at the point 21.The increased speed in the transition part 19 compensates to a certaindegree for this difference. However, compensation is somewhat below100%, as a compensation of 100% or more could entail the risk that thesludge cake might be pulled to pieces, which may result in a breakthrough of the light phase under and past the baffle 8.

The increased speed also compensates for the reduced cross-section areaof the conical part of the bowl 2 in the discharge part 18.

Though different embodiments of screw conveyors 3 according to theinvention have been described herein, said embodiments having differentcombinations of flight numbers and pitch angles and baffle types, itshould be understood that in particular flight pitch angles and type ofbaffle may be combined in any way within the scope of the invention.

1. A decanter centrifuge for separation of a supplied material in alight phase and a heavy phase, comprising an elongate bowl arranged forrotation about its longitudinal axis, said bowl having a separationchamber, a screw conveyor being provided in the separation chamber andbeing coaxial with the bowl, said screw conveyor comprising a body,which carries a screw comprising one or more flights and having anominal transport speed varying along the longitudinal axis, an inletwith at least one inlet opening in the screw conveyor for supply of thematerial to the separated, and at least one discharge opening for theheavy phase in the bowl at one end of the screw conveyor, in which thescrew conveyor is made to rotate relative to the bowl in view ofconveying the heavy phase towards the discharge openings for the heavyphase, and in which the screw conveyor is provided with a bafflepositioned between the inlet openings and the discharge openings, saidbaffle dividing the separation chamber in a substantially cylindricalseparation part and an at least partially conical discharge part, thedischarge openings for the heavy phase being positioned in the dischargepart, the inlet openings being positioned at the opposite side of thebaffle relative to said discharge openings, wherein immediately upstreamof the baffle, seen in relation to the transport direction, a transitionpart is provided between the separation part and the discharge part, andthe screw conveyor has a bigger nominal transport speed in thetransition part than in the separation part immediately before thetransition part, the change of the nominal transport speed of the screwfrom the nominal transport speed in the separation part immediatelybefore the transition part to the higher nominal transport speed in thetransition part being established by a change of the screw pitch.
 2. Adecanter centrifuge according to claim 1, wherein the change of thescrew pitch is abrupt.
 3. A decanter centrifuge according to claim 2,wherein the pitch angle of the screw in the separation part isconsiderably smaller than 45° relative to the tangential direction andthat the change of the screw pitch is an increase.
 4. A decantercentrifuge according to claim 2, wherein the pitch angle of the screw inthe separation part is considerably bigger than 45° relative to thetangential direction and that the change of the screw pitch is adecrease.
 5. A decanter centrifuge according to claim 1, wherein thechange of the screw pitch is gradual.
 6. A decanter centrifuge accordingto claim 5, wherein the pitch angle of the screw in the separation partis considerably smaller than 45° relative to the tangential directionand that the change of the screw pitch is an increase.
 7. A decantercentrifuge according to claim 5, wherein the pitch angle of the screw inthe separation part is considerably bigger than 45° relative to thetangential direction and that the change of the screw pitch is adecrease.
 8. A decanter centrifuge according to claim 1, wherein thepitch angle of the screw in the separation part is considerably smallerthan 45° relative to the tangential direction and that the change of thescrew pitch is an increase.
 9. A decanter centrifuge according to claim8, wherein said increase is 40–80%.
 10. A decanter centrifuge accordingclaim 8, wherein the screw has the bigger nominal transport speed overat least ⅓×1/n of a turn before the baffle, n being the number offlights.
 11. A decanter centrifuge according to claim 1, wherein thepitch angle of the screw in the separation part is considerably biggerthan 45° relative to the tangential direction and that the change of thescrew pitch is a decrease.
 12. A decanter centrifuge according claim 11,wherein the screw has the bigger nominal transport speed over at least⅓×1/n of a turn before the baffle, n being the number of flights.
 13. Adecanter centrifuge according to claim 1, wherein the screw has thebigger nominal transport speed over at least ⅓×1/n of a turn before thebaffle, n being the number of flights.
 14. The decanter centrifuge ofclaim 13, wherein said bigger nominal transport speed is more than about⅔×1/n of a turn.
 15. A decanter centrifuge according to claim 13,wherein the inlet is placed upstream of the transition part in theseparation part.
 16. A decanter centrifuge according to claim 1, whereinthe inlet is placed upstream of the transition part in the separationpart.
 17. A decanter centrifuge according to claim 16, wherein thebaffle has an axial extension, the border between the discharge part andthe transition part being positioned at the centre point of the axialextension of the baffle.
 18. A decanter centrifuge according to claim 1,wherein the baffle has an axial extension, the border between thedischarge part and the transition part being positioned at the centrepoint of the axial extension of the baffle.
 19. A decanter centrifugeaccording to claim 18, wherein the screw pitch is increasing in theseparation part in a direction away from the transition part.
 20. Adecanter centrifuge according to claim 1, wherein the screw pitch isincreasing in the separation part in a direction away from thetransition part.